Method for preheating lens making materials



Dec. 8, 1942.

EEE

PREH'AT/NG c. v. sMxTH Erm.

MTHOD FOR PRE-HEATING LENS MAKING MATERIALS Filed sept. 5. 1940.

l//V/T 0f INCREASE MPM/Www I I l l /NVENTR LENS 'ELAN/r m15 flu/nfs vsn/Ty PRESSURE FRA/ll( f. WIM/AHS Arron/Ey;

Dec. s, 1942. c. v. SMITH 'ETAL 2,304,663

METHOD Fon PREfHEATING LENS MAKING MATERIALS Eimer' Fired sept. 5, 1940 2 sheets-sheet 2 r gi/9% L@ nom/Na Patented Dec. 1942 :METHOD FR PREHEATING LENS MAKING v MATERIALS Charles V. Smith and Frank P. Williams, Dayton, `hio, assignors to The Univis Lens Company,

Dayton, Oho,\a corporation of Ohio Application September 5, 1940, SerialNo. 355,490

9 Claims.

This invention relates to a method for producing unbreakable lenses from synthetic materials.

When producing lenses from synthetic materials, by deforming a blank of synthetic resin between forming dies, the usual practice hereto,- fore has been to heat the blank of synthetic resin While within the 'forming press. This arrangement entails a long cycle of operation of a press in which a lens is to be formed, the cycle of operation including the entire period of heating, forming and subsequent cooling. Since the synthetic resins absorb heat relatively slowly,

the heating period of a blank of synthetic resinV has required relatively prolonged periods of time. The press during this time is retained from operation and the process is extremely uneconomical.

a method for forming lenses from synthetic resin wherein a pre-heated pair of forming dies, with a blank of resinous material therebetween, is

positioned within an unheated press, the residual The usual procedure heretofore followed ir forming lenses from synthetic materials has been to position the forming dies securely within a press. The dies were heated, by any suitable means, to the temperature at which it was de- -sired to work the blank'4 of resinous material Within the dies. A blank of resinous material, of suitable size and thickness, was positioned between the forming faces bf the forming dies.

These faces were provided'with surfaces having .an optical curvature of the desired shape which were arranged to cooperate to form the lens from the blank of resinous material positioned therematerial until it conformed with the optical curvature of the dies. The dies were then cooled sumciently to set the lens blank to altered shape. This entire forming operation has heretofore been carried out upon a press which resulted in relatively long periods of idle operation of the press for lens formingr purposes.

It is thus an object of this invention to provide a method for forming a blank of resinous material having optically curved surfaces by preheating the materialbefore placing the same in the forming press.

Another object of the invention is to provide a method for forming lenses from blanks of synthetic resinous material by pre-heating the blanks and the forming dies to a common elevated temperature, and subsequently/placing the forming dies -with a lens blank therebetween within a press forYI applying forming .pressure upon the dies. j

It is another object of the invention to provide heat of the dies retaining the blank Within the range of forming temperatures for a suilcient time to permit formation thereof.

Another object of the invention is to provide a method for forming lenses from synthetic resin wherein a pair of forming dies are preheated with a lens blank therebetween as a unit, .elevating their temperature to a common level and subsequently pressing the preheated unit within a forming press.-

Another object of the invention is to preheat a pair of forming dies with a resinous lens blank therebetween to a uniform elevated temperature and to subsequently apply pressure upon the dies for forming the resinous blank into a shape conforming with the optical curvature of the dies by deforming the blank at a rate which will not produce deleterious .optical strain in the formed. blank.

Another object of the invention is to provide a method for pre-heating a blank of 'resinous material wherein the die surfaces are retained in spaced relation with a resinous lens blank therebetween during pre-heating.

A still further object of this invention is to provide a method for forming lenses from synthetic resin wherein a pair of dies having optically curved surfaces thereon are positioned in spaced relation so that a resinous blank of material can be inserted therein without having the die faces engage the same, whereupon the die elements and the lens blank can be elevated'in temperature as a unit, and can be subsequently placed within a press for applying pressure upon the die for forming the resinous blank of material.

Another object of the invention is to provide a process in which a.-blank of resinous material can be pre-heated within means for permitting angular displacement of the optically curved surlfaces. of suitable dies to permit the optical surfaces thereof to assume proper relation with wespect to each other.

articles wherein the forming press is not heated.'

This invention consists of a method for forming blanks of synthetic resin into a conguration having optically curved surfaces.

In general, the method consists of positioning a blank of resinous material between die members which have optically curved surfaces appropriated thereon. These die members are retained in spaced relation during a pre-heating or temperature elevating period. A pair of die members with a resinous blank therebetween, comprises a unit which is positioned within an atmosphere maintained at an elevated temperature. The die elements and the resinous blank are elevated in temperature uniformly within the atmosphere, which atmosphere preferably brings the temperature of the resinous blank and dies to within a range of temperatures at which the lens blank can be altered in its physical shape.

Upon the dies and blank of resinous material reaching a determined temperature, or Within a range of temperatures, the unit, consisting of the dies and blank, is removed from the high temperature atmosphere and positioned within a press. 'I'he press will apply pressure upon the dies to cause the same to engage the blank of resinous material, whereupon the blank will assume the configuration of the optical curvature of the die surfaces.

To produce a satisfactory optical article from a blank of resinous material, we have determined that there 4is a specific rate of deformatiorhfor each temperature at which the resinous blank is worked. In general, the rate of deformation is greater as the temperature of the resinous material is increased. The rate of deformation of a blank of resinous material Whichwill not destroy its optical properties is determined by several factors. It is well known that all plastic materials have plastic flow when pressure is applied to the material, either tending to deform the same, or to stretch or compress the same. It has also been determined that a plastic material has a rate of plastic flow throughout the deformation range, beyond which ther'e is a Physical degradation of the molecular structure of the resin. e

The rate of deformation of a plastic is related to both the plastic flow and the elasticity. The elasticity is higher during the early stages of deformation of a plastic material, but rapidly tapers oi as the material is further deformed. The plastic flow of the material is relativelyslow,

hence, when the rate of deformation goes beyond the rate of plastic the rate of deformation must slow down until the plastic flow can meet the newly formed altered condition. The point at which the deformation slows down to a substantially constant rate determined by the rate of plastic flow, produces a rate of deformation beyond which plastic iiow is forced, whereby optical strain is produced. A

These optical strains may not be of such a value as impair the physical properties of the material, but are of such value that the strains produced displace the optical paths through the plastic material. By controlling the rate of deformation to control the value of the optical strain, the pl'wsical properties of the material are not impaired by the mechanical stress frozen in the material when set, which normally cause the material to become more soft, more susceptible to shatter, and loss of chemical resistance. It is this displacement of optical paths,.resulting in bi-refringence, which impairs the optical properties of a lens.

As heretofore stated, the rate of deformation is determined by the temperature of the plastic material, and it can be seen that a relatively accurate and uniform temperature and control of the material is desirable.

Hence, in this invention we provide an oven or enclosure IU which may be insulated for retaining heat therein. The enclosure I il is provided wlth a plurality of heating coils II through which a heating fluid may circulate. While we mention the heating fluid with regard to means for heating the enclosure I0, it is recognized that any means can be used for heating the interior of the enclosure I0. The heating means should be under control of a suitable controlling device for regulating the temperature of Y the enclosure IIJ.

In the present instance, a fluid tank l2 is provided. 'I'hs uid tank I2 is arranged with a suitable heating source therein, which is automatically controlled for maintaining the temperature of the fluid therein relatively constant. The heating source Within the uid tank I2, and the controls for the same, are well known in the art and further description thereof is not deemed necessary. A fluid pump I3 is connected to the fluid tank I2 and withdraws iluid therefrom through the conduit I4 and discharges same through the conduit I5.

The conduit I5 is connected to a conduit I9 which communicates with a mold carrying member I'I of the press 20. The mold or die carrying member l1 forms the lower half of a cooperating mold and die combination. The upper mold or die carrying member I6 is connected to the other mold or die carrying member Il by means of a uid circulating conduit'ls. This conduit I8 permits the hot circulating fluid to be circulated through the die carrying members I6 and I1 in series and discharge from the upper die carrying member I6 through the discharge conduit 22. The fluid from the pinnp discharge conduit I5 also circulates through the conduit I9 connected with the heating coils I I positioned with the heating enclosure Il). The fluid returns to the tank I2 from the heating conduits Il through the conduit 2|. While the heating circuits for the die carrying members IB and I'I and the enclosure I0 have been described as being in parallel relation, it is also possible to arrange the circuit as a series circuit if desired. Y We have heretofore .mentioned the fluid tank 12 with regard to a heating system, the heating fluid being circulated as heretofore described. It may also be considered that the tank I2 may contain apparatus for cooling a fluid for circulating through the mold carrying members I6 and I1 for cooling the same during certain cycles of operation in the formation of a lens from a blank of resinous material. Under normal conditions, however, the die or mold carrying members IB and I1 will cool sufliclently'rapidly after Athe heating uid has been stopped in circulation escasas Y 3 to be removed therefrom within a reasonable period of time.

A controlvalve 25 is positioned in the return conduit 22 from the mold or die carrying member I6, while a control valve 26 is provided in the 5- return conduit 2| from the heating coil of the enclosure I0. The uid circulating through either the mold carrying members I3 and Il or through the heating coil may thus be individually controlled to control the quantity of "fluid flowing therethrough and thus the temperature thereof. This manner of temperature con- `trol of the die carrying members VIt and, |'I and heating coil is in addition to an automatic control of the temperature of the fiuid within the 'tank I2.

The press 20, for forming the pressing or forming operation, is of conventional construction and ormold carrying member is suitably positioned and carried upon the bed'34 of the press. The press controls and actuating elements are Well known in the art and further description of the same is not believed necessary.

The interior of the enclosure I0 is maintained 3 at an elevated temperature, considerably above room temperature, by means of the heating coil I. A plurality of pairs of dies 35, and a plurality of blanks 36 of resinous material are adapted to be placed within the enclosure I 0 for elevating 3 the temperature thereof. When molding a large number of lenses upon a press, it is of particular advantage to have the die members and the lens blank heated to within the range of molding temperatures at which the lens blanks are to be Worked. The pre-heating of the dies and the blanks of resinous material eliminates'the necessity of heating the blanks of resinous material after being positioned between the dies and within the press. The lens blanks 35 and the dies 35 4' are completely surrounded by the high temperature atmosphere within the enclosure 13, hence, the heating of the dies 35 and the blanks of resinous material 36 will save considerable time by heating the blank within the dies before be- -5 ing positioned within the press.'ff,-.jFurther, the press can then be used for` forming operations only, and thus be in substantially continuous operation, since there is no needof waiting for an increase of temperature of he lens blanks 36 im r prir to the formation thereof.

'Ihe heating of the die carrying members I8 and by the fluid circuit heretofore described, rel tains the dies 35 at molding temperature through'- out the molding operation, hence, the temperature of the blank of resinous material is maintained substantially constant.

As heretofore mentioned, the deformation rate of a blank of resinous material is predetermined according to the temperature at which the blanks are to be worked. This deformation rate is of a determined order and cannot be greater than a and are uniformly heated, the maximum temperature ofthe same can be controlled. The die members 35 and the blank 36 can be removed from the interior of the enclosure I0 and the blank 36 be positioned within the mold 35. This unit, comprising. the die elements 35 and the blank 36 is then positioned within the press 20 at a controlled temperature. Y

The temperatures of the blank 36 and die elements 35 are indicated by the lines 40 and 4| respectively, (see Figure 2). It may thus be seen, by referring to the chart of Figure 2, that" the first step of pre-heating brings the die elements and blank up to molding temperature. The preheated unit'may then have pressure applied upon y the die elements by the press 20. The rate of increasing pressure application by the press 20 upon the die elements 35 has been predetermined to be the proper rate for forming the pre-heated lens blank at the temperature at which it is to be worked. This increasing rate of pressure application can be controlled either manually or automatically, however, the rate never exceeds the predetermined rate. During the forming operation, the pressure is gradually increased upon the die elements 35 as indicated by the line 42 of the chart of Figure 2.

Upon completion of the forming operation, the die elements 35, which' have been positioned inthe press 20 and retained yat molding tempera- `ture by the uid circulating through the die carrying elements I6 and mal7 be cooled either by the circulation of fluid through the mold carrying members I6 or I1, or by closing the valve 25 which stops circulation of heating fluid. The

cooling of the die and blank of resinous material causes the lens blank to set in its altered shape,

and may thereafterx be removed from the dies upon release of pressure by the press 20.

As heretofore described, the die elements and the blanks of resinous material were pre-heated within an atmosphere at elevated temperature, as individual units, and were subsequently placed in their proper relation prior to the insertion of the thus assembled unit into the press. To insure an absolute and uniform heating of the die elementsandthe blank of resinous material, we prefer to assemble the die elements and a blank of resinous material as units prior to pre-heating, and pre-heatsuch assembled elements as a unit. Further, we'prefer toprovide an arrangementwherein a blank of resinous material 'is positioned between the optically curved surfaces of a pair of die elements, and to space the die elements in spaced relationship, the surfaces of the blank being under no pressure during the pre` heating thereof. We prefer to arrange the die elements in 'spaced relationship to prevent local overheating or the heating of local spots for a longer period of timethan other portions of the blank of resinous material positioned therebetween. If a blank of resinous material is positionedbetween dies having curved surfaces and the assembly ls preheated to elevate the temperature of the assembly to moldingtemperature and the dies are permitted to contact the resinous blank the points of direct contact between the blank and the dies are brought up to temperalture relativelysoon. These spotsare retained at the elevated temperature over a longer period of time than are those areas which are not contacted directly. We have found that these local hot spots haveresulted in .changes of the optical properties of the material, particularly in a change in the index of refraction of the material in the area of the hot spot."

To Aprovide an assembly which can be pre- ...rrrrrr of anyspeciflc resilient supporting means, but

heated as a unit, we provide a pair of die elements 45 and 45. These die elements 45 and 45 may have formed therein surfaces providing an optical curvature, which surfaces may be formed within the face of the dies, or may be provided in the nature of inserts 41 and 48 placed in the dies 45 and 46 respectively. These inserts are preferably of glass, which produces `the exacting surface finish required for optical surfaces.

` The -die elements 45 and 46 are adapted to be retained in spaced vertical relation by means of a sleeve `49 positioned therearound. This sleeve 49 is provided with over-lapping ends as indicated at 50, to permit a certain degree of expansion thereof. The dies 45 and 45 arepro- .vided with relatively accurate circumferences so that the sleeve 49 will axially align the centers of the die elements 45 and 46, and thus will align l bled with a blank of resinous material 52 positioned therebetween. The assembly of the die ,elements 45 and 45 is such that they are retained in spaced relation with respect each other so that the blank of resinous material 52 is not contacted by the curved faces of the dies over the area from which the lens or plano will be produced. The sleeve -49 isV tightened upon the die elements 45 and 45 by means of the band 5I, whereby the assembly may be positioned within an oven or heating enclosure Illa for pre-heat-V ing'.

The sleeve 49 encircles and encloses the annular space between the die elements 45and 45 after once assembled. Since the sleeve 49 remains upon the die elements 45 and 46v during the preheating andthe subsequent pressing or forming operation, foreign matter willbe excluded from within the annular space between the die elements which would tend to injure the "surface of the blank upon formation thereof, and

the die surfaces are retained out of contact with the resinous blank during the entire pre-heating operation.

The units comprising the dies 45, 45 and the blank of resinous material 52 are elevated to a determined ,temperature 'within the enclosure Illa. Since the enclosure Ila is retained at substantially a constant temperature'in a manner similar to the enclosure I0, the units may be rerather the resilient support 55 may take any form which will permit the`pressing platen 54 to displace itself angularly with respect the base 51. The upper pressing platen 53 is secured to the plunger 58 which extends from the power source for the press, and provides the means for applying pressure upon the pre-heated unit positioned between the platens 53 and 54. The platen 53 is secured to the plunger 58 by means of a. resilient member 5S, which may take the form of a block of soft rubber which is vulcanized to both the platen 53 and the plunger 58, whereby the platen is secured to the plunger.

The vertical axis of the resilient support 59 is arranged in alignment with the central vertical axis of the platen 54. The pre-heated die unit, containing a blank of resinous material,

Yis positioned between the platens 53 and 54 so that the central axes of the die members and 46 are in substantial alignment with the vertical `axis of ther plunger 58 and the platen 53.

As heretofore described, the die members 45 and 46 are held in spaced relation by means of the friction engaging sleeve 49. However. when the plunger 5S moves the platen 53, the die member 45 will be moved within the sleeve 49 to approach the die member 46, whereby the blank of resinous material 52 will be caused to assume the optical 'curvature placed on the faces of the die inserts 41 and 48.

The sleeve 49 retains the dies 45 and 45 in substantial axial alignment. However, there may be slight mis-alignment of the planes of the optical curvature of the die inserts 41 and 45. This mis-alignment of the inserts 41 and 48 may occur from various causes. When producing an article foroptic'al work which has parallel and equidistantly spaced optically curved surfaces,

the curvature of the die insert 41 will'tend to` l solid cylinder, will permit the die 45 to align ittained within -the enclosure for any period oi' Y time to insure a uniform heating of the dies and blank of resinous material.

The pre-heated unit upon reaching its ultimate temperature can be removed from the pre-heating enclosure I0 and be positioned between th pressing platens of a press,

The pressing platens 53 and 54 can be heated by any suitable means, such as the huid circulating passages 55 (see Figure 3). 'I'he pressing platen 54 is preferably positioned upon a resilient support member 56 which in turn is supported by the press base 51. The resilient support member 55 may take the form of :a block oi' soft rubber or a plurality of springs may be positioned between the pressing platen 54 and the base 51 to resiliently support the platen 54.

self with the die 4s since the ends or the sleeve 49, as represented at 50, mayslide with respect each other to permit slight angular displace-v ment of the dies. 1

The arrangement justdescribed is particularly useful when producing an optical article having parallel and equidistantly spaced surfaces of optical curvature or can be used when producingy lenses to prevent the production of prisms in the finished product due to slight mis-alignment of the die faces.

After the unit, consisting of the dies 45 and 45 with a lens bla'nk 52 therebetween, has been 'pre-heated within the enclosure Illa to a deterf the blank 52 and theldies 45and 46- can be retained for an'indefinite' period of time, and

rto be the proper rate not to produce optical We do not wish to limit ourselvesrto the use 1l Strain within the finished lens. any event.

the molding temperature of the blank 52 is retained throughout the complete formation period thereof. y

However, there .are certain types of lenses which do not require that the molding temperature be retained for so long a period, particularly those lenses which require very slight formation. In these instances it would not be necessary to heat the platens 53 and -54, since the residual heat of the'pre-heated dies 45 and 48 is sufcient to maintain the blank 52 within the molding temperature range for the period of time re- Aquired for forming. The determining factor as to the length of time the blank 52 will be retained within its molding temperature range is the mass of the dies 45 and 46. It may readily be seen that the larger the mass of material from which the dies 45 and 46- are made, the greaterthe amount of residual heat which will be retained from the pre-heating step. It is quite possible that if the mass of the dies 45 and 46 is suiliciently large, the heating of the platens 53 and 54 may be entirely dispensed with when form-i ing any optical article.

As specific examples of optical articles'which can be formed without the necessity of heating the pressing platens 53 and 54, lenses of slight correction or optical planos can be -cited as the 'specific examples. When forming this type of optical articles, the movement of the mass of the blank of resinous material is very slight and hence the Atime required for molding is.consid.

. same general procedure must be followed in that erably shorter than for lenses of relatively high Y correction. When forming optical articles of these types by pre-heating the blank of resinous material and the forming dies as a unit, it has been found that the residual heat is sufhcient to permit the completel formation of the article. Thus, under these cases the pressing platens 53 and 54 need not be heated and may actually be cooled. y

As an example of the method of the steps in forming a lens of slight correction or an optical plano, the curve chart of Figure 6 may be referred to. In this chart th'e lines 60 and 5I' represent the temperatures of the die halves and the blank of resinous material respectively. As

can be seen from this chart the temperature of the die halves and the lens blank is raised to the molding or forming temperature during the pre-heating operation. y

Upon the mold and blank reaching the forming temperature they are removed from the heatving voven and are then positioned in a forming press. Since the degree 'of correction for the optical article is `1very slight, the molding or forming pressure can be applied to the article relatively rapidly, as indicated by the line 62, whereby the forming timelis reduced considerably. Also, since the forming pressurecan be 'applied at a. rapid rate, the necessity for maintaining the blank of resinous material at forming temperature over a prolonged period'of time is eliminated. Hence, the pressing vplatens need not be'heated and actually may be cooled, where by the temperature of the blank and forming dies begins an immediate fall upon being positioned within the forming press. v

The forming pressure will be retainedl upon the blank of resinous material until the blank has cooled sufliciently to set, at which time, the forming pressure can be released.

the material cannot be deformed at a rate which produces optical strain, but since the articles of this type have such slight correction, the degree of formation is such that they can be moved more rapidly than can lenses of high correc-- tion. r

It can be seen from the foregoing description that the method and apparatus of this invention has many advantages. The pre-heating of the dies and lens blanks, either separately or as complete units of assembly, permits the blanks of resinous material to be heated uniformly throughout the entire mass thereof. The blanks of resinous lmaterial can be pre-heated in large quantities and retained at a satisfactory formin g temperature until ready for use. This is also true of the die elements. Further. it may readily be seen that the pre-heating of the dies and blank of resinous material is capable of permitting the elimination of the heating of the press platens. Also, the arrangementve have provided for securing the die elements and 46 together during the entire pre-heating and forming operations provides an arrangement for axially aligning th'e diemernbers. Under circumstances wherein it is desired to produce a plano lens or an optical article having equidistantly spaced opticalcurvatures, the resilient pressing means as well as the resiliency of the sleeve 449, will permit parallel alignment of the surface of optical curvature of the dies. When producing optical curves on both sides of a blank of resinous material the resilient supportof the dies will permit the curved faces thereof `to attain proper positions relative each other to prevent production of prisms.

While the form and embodiment disclosed and described constitutes a preferred form, yet it may readily be understood that we do not wish to lnnit our invention to the specic forms described but that the invention may be practiced by the use of any number `of mechanical arblank is to be formed before applying any pres- It may thus 'be seen that the manipulation of A lenses of slight correction, or planos, may be material which consists, of preheating` sure upon the blank, and of applying closing -pressure upon said mold while the temperature of the blank is maintained substantially constant at the predetermined forming temperature dur. ing the forming period to form the blank to the conguration of the mold.

2. A method for forming an optical article havlng an optical surface from a blank of resinous blank and forming mold uniformly thr the mass of the mold and blank to onegqpd the same predetermined temperature at which the blank is to be formed beforeapplying any4 pressure upon the blank, of applying closing pressure upon said mold while the tenii'erature of the blank is maintained substantially nnstant at the predetermined forming temperature during the forming period to form'the blank to the configuration of; the mold, and of cooling the mold while con- 6 2,so4,ees

predetermined temperature at which the blank is to be formed before applying any pressure upon the blank, and of applying closing pressure upon said mold while the temperature of the blank is maintained substantially constant at the predetermined forming temperature during the forming period to form the blank to the confiruration of the mold, said pressure being apzpon'the mold to cause movement of the in i." the blank at the rate which does not procs.A nptical strain in the blank suincient to induce Tsi-zefringence. l

4. A method for forming an optical article having an optical surface upon a blank of resinous material which consists, of positioning a blank of resinous material between die halves having optical curvatures on the faces thereof, of heating the lens blank and the die halves as a unit within an enclosure maintained at a constant predetermined temperature suinclently high to plasticize the blank for satisfactory physical alteration of the blank until the predetermined temperature of the` enclosure exists `throughout the mass of the unit, of positioning the heated unit in a heated press to maintain the temperature of the unit constant, and of applying closing pressure upon the unit to form the blank to the configuration of the dies.

5. A method for` forming an optical article having an optical surface upon a? blank of resinous material which consists. of positioning a blank of resinous material between die halves exists throughout the mass of the unit. of positioning the heated unit in a heated press to maintain the temperature of the unit constant. and of applying closing pressure upon the unit to form` the blank to the configuration of the dies.

6. A method for forming an optical article having an optical surface from a blank of resinous material which consists. of positioning a blank of resinous material between die halves having optical curvatures on the faces thereof retained in spaced relationship with respect to each other; and out of engagement with the optical area of said blank, of preheating the 'Y high to plasticize the blank ,for satisfactory physi- 65 article.

cal alteration of the blank until the predetermined temperature of the enclosure exists throughout the mass of the unit, of supporting the preheated unit in a press between resilient members that allow freedom of movement of the die halves angularly with respect to the axis of the die halves, and of applying closing pressure upon the unit to cause the curved optical faces of the die halves to engage the surfaces of the blank and arrange themselves in parallel relationship by uniformly distributing the plastic material withinthe blank between the die surfaces when forming the blank to the configuration of the die surfaces.

'7. A method for forming an optical .article having an optical surface upon resinous material which consists, of preheating a lens blank and forming mold as a unit within an enclosure maintained at a constant predetermined temperature suillciently high to plasticize the blank for satisfactory physical alteration of the blank until the predetermined temperature of the enclosure exists throughout the mass of the unit, of positioning the preheated unit in a cool press, and of applyingclosing pressure upon said mold while the blank is therein to form the blank to the configuration of the mold before the temperature of the blank falls from within the forming temperature rangemaintained by the residual heat of the mold during thel forming operation.

8. A method for preparing a blank of resinous material for formation into an optical article of high precision optical surfaces, of positioning the4 surfaces of the dies out of engagement with the optical area of the blank during the heating thereof kto prevent overheating of local areas in the optical area, and of heating the blank and the dies as a unit to a uniform predetermined temperature within an enclosure maintained at a constant predetermined temperature sufficiently high to plasticize the blank for satisfactory physical alteration of the blank until the predetermined temperature of the enclosure exists throughout the mass of the unit.

9. A method for preparing a blank of resinous material for formation into an optical article of high precision ,which consists, of placing a solid blank of resinous vmaterial between dies having high' precision optical surfaces, of positioning the surfaces of the dies out of engagement with the optical area of the blank during the heating thereof to prevent overheating of local areas in the optical area, of heating the blank and the dies as a unit to a uniform predetermined temperature within an enclosure maintained at a contant predetermined temperature suillciently high to plasticize the blank for satisfactory physical alteration of the blank until the predetermined temperature of Ithe enclosure exists throughout the mass of the unit, and of storing the unit in the enclosure until it is to be used in a press for forming the blank'into an optical CHARLES V. SMII'H. 'FRANK P; WHLIAMS. 

